Question

Innocent until proven guilty? In Japanese criminal trials, about 95% of the defendants are found guilty. In the United States, about 60% of the defendants are found guilty in criminal trials. Suppose you are a news reporter following four criminal trials.

a) If the trials were in Japan, what is the probability that all the defendants would be found guilty? What is the probability if the trials were in the United States?
b) Of the four trials, what is the expected number of guilty verdicts in Japan? What is the expected number in the United States? What is the standard deviation in Japan? What is the standard deviation in the United States?
c) As a U.S. news reporter, what is the smallest number of trials n you would need to cover to be at least 99% sure of two or more convictions? What is the smallest number of trials n you would need if you covered trials in Japan?

Answer 1

Explanation:

Let X be the number of guilty defendants in Japan

Let y be the number in the US

we have n = 4

p = 0.95

a.) prob of trials in Japan

p(x=4)

= 0.95⁴

= 0.8145

prob of trials in US

p(X=4)

= 0.60⁴

= 0.1296

B. expected number in Japan

E(X) = n*p

= 4*0.95

= 3.8

we find the variance

= n*p(1-p)

= 4*0.95(1-0.95)

= 0.19

standard deviation for japan

= √0.19

= 0.4359

expected number for US

E(X) = n*p

= 4*0.60

= 2.4

we find the variance

= n*p(1-p)

= 4*0.60(1-0.6)

= 0.96

standard deviation for US

= √0.96

= 0.979

c. 2*4 = 8